The embodiments of the present invention relate to communication field, and particularly relate to a method of performing target Raman gain locking and a Raman fiber amplifier.
Distributed Raman fiber amplifier has been widely applied in communication system now. Since gain medium of the distributed Raman fiber amplifier is transmission fiber itself, input power into a Raman pumping module cannot be monitored in real time. Signal power into the Raman pumping module can be monitored only in case that pump is powered OFF. If a pump laser the Raman pumping module is in a ON state, signal power into the Raman pumping module is amplified power, that is, the pumping module cannot detect both of the input power in case that pump is powered OFF and the signal power in case that pump is powered ON. Therefore, the gain control in the distributed Raman fiber amplifier cannot be performed as conventional EDFA that performs the gain control by comparing input and output powers.
In past, input power of the distributed Raman fiber amplifier is small mostly. Since a power of a saturated input signal is high, the Raman fiber amplifier in past mostly operates in linear amplification region of small signal. As long as the power level of the pump is fixed, gain is nearly constant as input power varies in small signal region. However, since broadband is growing fast, demand for high bandwidth is increasing, thus operators upgrade number of wavelengths continuously. As a result, power into the Raman fiber amplifier is greatly increased, the Raman fiber amplifier operates at knee point of linear gain region and saturated gain region. Thus, in order to avoid influence on the gain by uplink and downlink channels, it is necessary to introduce Raman gain locking. Also, in practice of the distributed Raman fiber amplifier, different user may deal with different kinds of optical fibers such as SMF-28, Leaf; Truewave fibers and so on. Fibers of same kind may even have different attenuation coefficients due to different batches, which result in large gain error when pump power is constant or drive current is constant. In consideration of influence by aging over time and change of ambient temperature of fiber, loss factor of the fiber become larger, thus Raman gain is decreased. During engineering installation, different node loss may cause large gain error (pump power is constant). During construction, cross parts with different length may exist. For flexible application, variable Raman gain is requirement for system. In view of above, it is necessary for system to perform gain locking on Raman fiber amplifier and realize adjustable gain.
Erbium Doped Fiber Amplifier (EDFA) realizes amplification of signal by energy level transition. Thus, ASE of EDFA is greatly influenced by absence/presence of signal input. However, in the distributed Raman fiber amplifier, amplification process is realized by nonlinear effect stimulated Raman scattering without energy level transition, in which ASE power level of the distributed Raman fiber amplifier is irrelevant of absence/presence of signal light. In case of fixed Raman gain, ASE power level of the Raman fiber amplifier will remain unchanged. Since ASE in operation band is superposed with signal, in-band ASE power cannot be monitored, while out-of-band ASE power can be monitored. Therefore, Raman gain locked can be realized by controlling power level of out-of-band ASE optical signal. In order to obtain different groups of Raman gains, it needs to obtain different groups of out-of-band ASE power levels. Thus, the distributed Raman fiber amplifier with adjustable gain is technologically practical.
Chinese patent CN1412616A disclosed a method of performing gain locking by ASE, in which a combination of several BPFs and photoelectric detectors for detecting around different wavelength is added at 5% of detecting position of signal light, to detecting ASE optical signal power at different frequencies. Then dynamically control in real time of Raman gain of the Raman fiber amplifier can be realized by controlling the magnitudes of ASE optical signal powers on those points. The method has two disadvantages on value-sampling of ASE as following: 1) if the value is sampled on operation wavelength, bandwidth is not broad enough, thus obtained ASE power value is relative small, accuracy of gain may be greatly influenced by dark current of PIN; and 2) if the ASE value is sampled on operation wavelength, although BPF is standard channel of non ITU-T, ASE power may be greatly influenced by SSE of light source itself; in particular, for 40 Gbit/s and 100 Gbit/s signals modulated with DPSK and with broad signal bandwidth, much of base SSE power may interfere with value-sampling region of ASE; further, it cannot be avoided that ASE power will be generated from EDFA in system cascading, which cannot be prevented. Above two disadvantages may limit the practicality of the technology. Moreover, in said patent, gain locking is performed only on a fixed gain, it cannot realize variable gain.
Chinese patent CN101552428A disclosed a technology and device for realizing signal power detection and ASE compensation according to ASE power outside the operation band and ASE and signal powers within the operation band, in which detection of in-band ASE power is performed by means of power of out-of-band ASE optical signal, thus monitoring of signal power is realized, while ASE power is not used for gain locking and adjustment. Further, in the solution, influence on entire signal power detection by out-of-band ASE light of EDFA itself is not considered, in that the main object of the patent is to solve the problem of pump off in case of fiber breakage or absence of signal light. Out-of-band ASE light of EDFA itself in the system has little influence on it, because in case of fiber breakage or absence of signal light, EDFA itself does not output ASE power.